At the outset, it will be pointed out that the invention relates to constructional features and assembly techniques for motors rather than the specific electromagnetic mechanism which drives them. As a result, the present invention can be applied to a number of different motor types including switched reluctance motors, permanent magnet brushless motors and hybrid stepping motors among others.
Chai et al. U.S. Pat. No. 4,029,977 shows a relatively conventional construction for a variable reluctance stepping motor. It is seen that the motor includes an external case machined to register with a pair of end bells which also must be machined to register with both the case and the rotor bearings. The external case, in turn, serves to register a wound and insulated stator lamination stack. Thus, when the elements are assembled, the rotor is held in its bearings in the machined surfaces in the end bells, the numerous registration devices assure that the rotor is properly positioned within the stator. However, such an arrangement carries with it a degree of manufacturing expense because of all of the separate parts must be fairly precisely machined in order for all of the registration systems to function in properly aligning the rotor within the internal bore of the stator.
It has been proposed to produce a motor without an external case, exposing the exterior of the stator laminations. One such approach is illustrated in Kawada et al. U.S. Pat. No. 4,538,084, and another in Kawada et al. U.S. Pat. No. 4,626,725. The former uses threaded rods welded within the stator lamination assembly in order to appropriately register the end caps to the stator. The latter uses threaded rods of two lengths, one interfitting into the end caps and the other on which the end caps bottom in order to maintain registration of the end caps to the stator. While those approaches save the expense of the motor housing, they add complication in the form of such elements for attaining and maintaining registration of the end caps (and therefore the rotor) to the stator.
It is also applicants' understanding that a motor of the general type illustrated in the aforementioned Chai et al. patent has been configured without a case, that is, with the stator laminations exposed intermediate a pair of end caps. As applicants understand it, in order to achieve the elimination of the case, the end caps were altered in two ways. First of all, tapered tabs were formed on the periphery of the end caps, projecting toward the lamination stack at three points around each end cap in order to accurately register the end caps with the lamination stack. Secondly, apertured mounting ears were also formed on the end caps, protruding at right angles from the tabs. The mounting ears received bolts which spanned the lamination stack between end caps to firmly secure the end caps to the lamination stack, thereby not only providing register but also rigidity. In that registered rigid configuration, means were then provided to machine the bore which extended through the end caps and stator assembly. A multi-station lapping machine was used to simultaneously machine bearing surfaces in the end caps and the stator bore. The rotor was then positioned in the machined aperture with the rotor bearings mounted in the end caps and the rotor free to rotate within the stator.
That approach suffers from certain problems. First of all, the exterior tabs and mounting ears on the end caps require the envelope occupied by the motor to be enlarged; in many applications where space is at a premium the enlarged size could be unacceptable. More importantly, machining the bearing surfaces and stator ID with the stator fully assembled results in machining debris entering the motor. The motor was constructed of open configuration, providing large apertures in the end caps which allowed the motor to be thoroughly washed in an effort to remove the machining debris. In addition, machining debris could be trapped within the coils, fall between the end caps and the stator or otherwise lodge itself in apertures in the stator assembly, and that machining debris could work loose during later operation of the motor to cause premature failure.
The stator machining problem could be particularly severe for motors of the "enhanced" type which have permanent magnet segments interposed between rotor teeth, since machining of the rotor bore would then create machining debris which included highly magnetic particles. The magnetic particles would be extremely difficult to remove by standard washing techniques and motor stators were sometimes varnished or otherwise internally coated to secure the magnetic particles which could not be removed in position. Thus, there was created a significant cleanup problem and the possibility of motor failure by means of unremoved machining debris.
One of the problems which has necessitated relatively elaborate registration devices is thermal cycling of the motor in the case where the end caps have a significantly different coefficient of thermal expansion than the stator laminations. The stator laminations are typically steel. The end caps, however, must be nonmagnetic and often made of aluminum which expands much more than the steel when the motor is operating and dissipating heat, and contracts much more than the steel when the motor is at rest and cools. If the end caps are not held in rigid and fixed register with respect to the stator laminations, when the motor is cyclically heated and cooled during operation, the end caps can "walk" with respect to the stator, ultimately causing misalignment of the rotor within the stator.